Thin Film Surface Mountable High Frequency Coupler

ABSTRACT

A high frequency coupler is disclosed that is configured for grid array-type surface mounting. The coupler includes a monolithic base substrate having a top surface and a bottom surface. A first thin film microstrip and a second thin film microstrip are each disposed on the top surface of the monolithic base substrate. Each microstrip has an input end and an output end. At least one via extends through the monolithic base substrate from the top surface to the bottom surface of the monolithic base substrate. The via(s) are electrically connected with at least one of the input end or the output end of the first microstrip or the second microstrip. The coupler has a coupling factor that is greater than about −30 dB at about 28 GHz.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims filing benefit of U.S. Provisional PatentApplication Ser. Nos. 62/639,309 having a filing date of Mar. 6, 2018and 62/642,219 having a filing date of Mar. 13, 2018, which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

High frequency radio signal communication has increased in popularity.For example, the demand for increased data transmission speed forwireless smartphone connectivity has driven demand for high frequencycomponents, including those configured to operate at 5G spectrumfrequencies. A trend towards miniaturization has also increased thedesirability of small, passive components for handling such highfrequency signals. Miniaturization has also increased the difficulty ofsurface mounting small, passive components suitable for operation in the5G frequency spectrum. A compact, high frequency coupler that is easilysurface mounted would be welcomed in the art.

SUMMARY

In accordance with one embodiment of the present invention, a highfrequency, surface mountable coupler is disclosed. The coupler includesa monolithic base substrate having a top surface, a bottom surface, alength in a longitudinal direction, and a width in a lateral directionthat is perpendicular to the longitudinal direction. The couplerincludes a first thin film microstrip disposed on the top surface of themonolithic base substrate. The first microstrip has an input end and anoutput end. The coupler includes a second thin film microstrip disposedon the top surface of the monolithic base substrate. The secondmicrostrip has an input end and an output end. The coupler includes atleast one via extending through the monolithic base substrate from thetop surface to the bottom surface of the monolithic base substrate. Thevia(s) are electrically connected with at least one of the input end orthe output end of the first microstrip or the second microstrip. Thecoupler has a coupling factor that is greater than about −30 dB at about28 GHz.

In accordance with another aspect of the present invention, a method forforming a high frequency, surface mountable coupler is disclosed. Themethod includes forming at least one via through hole that extends froma top surface of a monolithic base substrate to a bottom surface of themonolithic base substrate. The method includes depositing each of afirst thin film microstrip and a second thin film microstrip on the topsurface of the monolithic base substrate. Each of the first and secondthin film microstrips are sized and spaced apart such that the couplerhas a coupling factor that is greater than about −30 dB at about 28 GHz.The method includes depositing a conductive via material inside the viathrough hole(s) to form at least one via electrically connecting atleast one of the first thin film microstrip or the second thin filmmicrostrip with a contact pad on the bottom surface of the monolithicbase substrate.

In accordance with another aspect of the present invention, a highfrequency, surface mountable coupler is disclosed. The coupler includesa monolithic base substrate having a top surface, a bottom surface, alength in a longitudinal direction, a width in a lateral direction thatis perpendicular to the longitudinal direction, and a thickness in a Zdirection that is perpendicular to each of the longitudinal directionand the lateral direction. The coupler includes a first thin filmmicrostrip disposed on the top surface of the monolithic base substrate.The first microstrip has an input end and an output end. A second thinfilm microstrip is disposed on the top surface of the monolithic basesubstrate, and the second microstrip has an input end and an output end.At least a portion of the first thin film microstrip and at least aportion of the second thin film microstrip extend parallel to each otherin a first direction along a coupling length that ranges from about 0.2mm to about 3.8 mm.

In accordance with another aspect of the present invention, a basestation circuit is disclosed. The base station circuit includes a signalsource component and a high frequency, surface mountable coupleroperatively connected with the signal source component. The couplerincludes a monolithic base substrate having a top surface, a bottomsurface, a length in a longitudinal direction, and a width in a lateraldirection that is perpendicular to the longitudinal direction. Thecoupler includes a first thin film microstrip disposed on the topsurface of the monolithic base substrate. The first microstrip having aninput end and an output end. The coupler includes a second thin filmmicrostrip disposed on the top surface of the monolithic base substrate.The second microstrip has an input end and an output end. At least onevia extends through the monolithic base substrate from the top surfaceto the bottom surface of the monolithic base substrate. The via(s) areelectrically connected with at least one of the input end or the outputend of the first microstrip or the second microstrip. The coupler has acoupling factor that is greater than about −30 dB at about 28 GHz.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedFigures, in which:

FIG. 1A illustrates a top down view of one embodiment of a thin filmcoupler in accordance with aspects of the present disclosure;

FIG. 1B illustrates a cross section view along section A-A in FIG. 1A;

FIGS. 2A and 2B illustrate perspective views of the embodiment of thethin film coupler illustrated in FIGS. 1A and 1B;

FIG. 3A illustrates a top down view of another embodiment of a thin filmcoupler including a cover ground plane according to aspects of thepresent disclosure;

FIG. 3B illustrates a section view along section A-A in FIG. 3A;

FIG. 3C illustrates a perspective view of the embodiment of the thinfilm coupler illustrated in FIGS. 3A and 3B;

FIG. 4 illustrates diagrammatic views of various surfaces of severalembodiments of the coupler in accordance with aspects of the presentdisclosure;

FIG. 5 illustrates a flow diagram of a method for making a highfrequency coupler configured for array-type surface mounting inaccordance with aspects of the present disclosure;

FIG. 6A illustrates a simplified, schematic view of an embodiment of acoupler having a transmission line and a coupled line; and

FIG. 6B represents theoretically calculated S-parameters for anembodiment of a coupler in accordance with aspects of the presentdisclosure across a frequency range extending from 18 GHz to 32 GHz.

Repeat use of reference characters throughout the present specificationand appended drawings is intended to represent same or analogousfeatures or elements of the invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

A surface mountable coupler device is provided that is particularlyuseful in high frequency circuits, including those operating in the 5Gfrequency spectrum. The 5G frequency spectrum generally extends fromabout 20 GHz to about 30 GHz. Couplers generally provide couplingbetween two signal lines without direct electrical contact. Exemplaryuses include radio frequency (RF) mixers, amplifiers, and modulators.For instance, couplers may be used to provide coupling for a feedbackcontrol loop or an amplifier output section in a RF transmitter.

A thin film coupler may be formed on one or more monolithic substrates.For example, in some embodiments, the thin film coupler may include abase substrate and a cover substrate. The substrate(s) may comprise oneor more suitable ceramic materials. Suitable materials are generallyelectrically insulating and thermally conductive. For example, in someembodiments, the substrate may comprise alumina (Al₂O₃), aluminumnitride (AIN), beryllium oxide (BeO), aluminum oxide (Al₂O₃), boronnitride (BN), silicon (Si), silicon carbide (SiC), silica (SiO₂),silicon nitride (Si₃N₄), gallium arsenide (GaAs), gallium nitride (GaN),zirconium dioxide (ZrO₂), mixtures thereof, oxides and/or nitrides ofsuch materials, or any other suitable ceramic material. Additionalceramic materials include barium titanate (BaTiO₃), calcium titanate(CaTiO₃), zinc oxide (ZnO), ceramics containing low-fire glass, andother glass-bonded materials.

In some embodiments, one or more of the substrates may comprise sapphireor ruby. Sapphire and ruby are types of corundum, which is a crystallineform of aluminum oxide (a ceramic material) containing additional tracematerials. A substrate comprising sapphire may provide several benefitsincluding excellent electrical insulation, heat dissipation, and hightemperature stability. Additionally, because sapphire is generallytransparent, internal features of the coupler may be visually inspected,reducing the time and difficulty associated with checking completedcomponents for quality.

The coupler may include various thin film components, including a pairof microstrips formed on a top surface of the base substrate and a baseground plane formed on a bottom surface of the base substrate. In someembodiments, the coupler may also include an additional thin film groundplane disposed on a top surface of the cover substrate. The coversubstrate may be arranged on the top surface of the base substrate.

The thin film components may have thicknesses of about 50 micrometers orless, in some embodiments 20 micrometers or less, and in someembodiments 5 micrometers or less. For example, in some embodiments thethickness of the thin film components may range from about 0.05micrometers to about 50 micrometers, in some embodiments from about 0.1micrometers to about 20 micrometers, in some embodiments from about 1micrometer to about 5 micrometers, e.g., about 3 micrometers.

The thin film components may be precisely formed using a variety ofsuitable subtractive, semi-additive, or fully additive processes. Forexample, physical vapor deposition and/or chemical deposition may beused. For instance, in some embodiments, the thin film components may beformed using sputtering, a type of physical vapor deposition. A varietyof other suitable processes may be used, however, includingplasma-enhanced chemical vapor deposition (PECVD) and electrolessplating, for example. Lithography masks and etching may be used toproduce the desired shape of the thin film components. A variety ofsuitable etching techniques may be used including dry etching using aplasma of a reactive gas (e.g., oxygen, chlorine, boron trichloride)and/or wet etching.

The thin film components may be formed from a variety of suitableelectrically conductive materials. Example materials include copper,nickel, gold, tin, lead, palladium, silver, and alloys thereof. Anyconductive metallic or non-metallic material that is suitable for thinfilm fabrication may be used, however.

Vias may connect the microstrips on the top surface of the basesubstrate with contact pads on the bottom surface of the base substrate.This may allow the thin film coupler to be surface mounted to a printedcircuit board (PCB), for example. In some embodiments, the vias may beformed by laser drilling holes through the base substrate and thenfilling (e.g., sputtering, electroless plating) the internal surfaces ofthe holes with a suitable conductive material. In some embodiments, thethrough holes for the vias may be filled concurrently with theperformance of another manufacturing step. For example, the vias may bedrilled before the thin film components are formed such that both thevias and the thin film components may be simultaneously deposited. Thevias may be formed from a variety of suitable materials including thosedescribed above with reference to the thin film components (e.g.,microstrips and ground plane).

In some embodiments, one or more protective layers may be exposed alongan exterior of the coupler. For example, the protective layer(s) may beformed over a top surface and/or a bottom surface of the coupler. Asused herein, “formed over,” may refer to a layer that is directly incontact with another layer. However, intermediate layers may also beformed therebetween. Additionally, when used in reference to a bottomsurface, “formed over” may be used relative to an exterior surface ofthe component. Thus, a layer that is “formed over” a bottom surface maybe closer to the exterior of the component than the layer over which itis formed.

A top protective layer may be formed over the top surface of basesubstrate and microstrips or over a top surface of the cover substrate(if present). A bottom protective layer may be exposed along a bottomsurface of the coupler, for example formed over the bottom surface ofthe base substrate. The bottom protective layer may be formed usingphotolithography techniques in a manner that leaves openings or windowsthrough which the contact pads may be deposited, for example byelectroplating.

The protective layer(s) may include a polymeric material, such aspolyimide, SiNO, A1203, SiO2, Si3N4, benzocyclobutene, or glass. In suchembodiments, the protective layer may have a thickness that ranges fromabout 1 micron to about 300 microns.

The thin film coupler may be configured to be surface mounted on a basesurface, such as printed circuit board (PCB), such that the bottomsurface of the base substrate is connected with the PCB. Specifically,the thin film coupler may be configured for grid array-type surfacemounting. For example, the thin film coupler may be configured for landgrid array (LGA) type mounting, ball grid array (BGA) type mounting, orany other suitable type of grid array-type surface mounting.

Regardless of the particular configuration, the present inventors havediscovered that through the selective control of the production of thethin film components and vias, a high frequency coupler can be achievedthat has a coupling factor that is greater than about −30 dB at about 28GHz and that is suitable for grid array mounting. For example, in someembodiments, the coupling factor may be greater than about −25 dB atabout 28 GHz, and in some embodiments greater than about −20 dB at about28 GHz, e.g., −19.9 dB.

The coupler may also be suitable for coupling across a broad range ofhigh frequencies. For example, coupler may have a coupling factor thatis greater than about −35 dB from about 18 GHz to about 32 GHz, orhigher, in some embodiments, greater than about −30 dB from about 18 GHzto about 32 GHz, and in some embodiments greater than about −25 dB fromabout 18 GHz to about 32 GHz. Additionally, in some embodiments, thecoupler may have a coupling factor that is greater than about −35 dBfrom about 10 GHz to about 70 GHz, in some embodiments, greater thanabout −30 dB from about 10 GHz to about 70 GHz, and in some embodimentsgreater than about −25 dB from about 10 GHz to about 70 GHz.

Additional performance characteristics of the coupler may be desirable,including the return loss, insertion loss, and isolation factor. Forexample, coupler may have a low return loss, which is desirable becausereturn loss represents the portion of the signal that is reflected backby the coupler. In some embodiments, the return loss may be less than−15 dB from about 10 GHz to about 70 GHz, in some embodiments less thanabout −20 dB from about 20 GHz to about 32 GHz, in some embodiments lessthan about −25 dB from about 21 GHz to about 32 GHz, and in someembodiments less than about −30 dB from about 22 GHz to about 32 GHz. Insome embodiments, the return loss may be less than about −35 dB at about28 GHz.

The coupler may have an insertion loss that is near zero, whichindicates that the signal is passed through a transmission line of thecoupler without being substantially affected. The insertion loss may begreater than about −1 dB from about 18 GHz to about 32 GHz, in someembodiments greater than about −0.75 dB from about 18 GHz to about 32GHz, and in some embodiments greater than about −0.6 dB from about 18GHz to about 32 GHz. In some embodiments, the insertion loss may begreater than about −1 dB from about 10 GHz to about 70 GHz, in someembodiments greater than about −0.75 dB from about 10 GHz to about 70GHz, and in some embodiments greater than about −0.6 dB from about 10GHz to about 70 GHz.

The coupler may also have an isolation factor that is less than about−25 dB from about 18 GHz to about 32 GHz, in some embodiments less thanabout −30 dB from about 18 GHz to about 32 GHz. Additionally, in someembodiments, the coupler may have an isolation factor that is less thanabout −25 dB from about 10 GHz to about 70 GHz, in some embodiments lessthan about −30 dB from about 10 GHz to about 70 GHz. In someembodiments, the isolation factor may be less than about −35 dB at about28 GHz, in some embodiments less than about −40 dB at about 28 GHz, insome embodiments less than about −45 dB at about 28 GHz.

The coupler includes a pair of microstrips disposed on a top surface ofa base substrate. At least a portion of the microstrips may be arrangedsubstantially parallel to each other along a coupling length. Thecoupling length may be selected to obtain the desired performancecharacteristics of the coupler, including the coupling factor. Withoutbeing bound by theory, the performance characteristics of the couplermay generally be optimized when the coupling length is equal to orproportional to (e.g., a multiple of) λ/4, where A is wavelength of adesired operating frequency of the coupler propagating through the basesubstrate and/or cover substrate. A coupling length of the microstripsmay be equal to or proportional to λ/4. Additionally, the length(s) ofthe via(s) may be equal to or proportional to λ/4.

The wavelength, λ, through a material having a dielectric constant,ε_(r), can be calculated as follows:

$\lambda = \frac{C}{f\sqrt{ɛ_{r}}}$

where C represents the speed of light in a vacuum, and f representsfrequency.

In some embodiments, the dielectric constant of the base substratematerial and/or cover substrate material may range from about 0.1 toabout 50 as determined in accordance with ASTM D2520-13 at an operatingtemperature of 25° C. and frequency of 28 GHz, in some embodiments fromabout 0.5 to about 20, in some embodiments from about 1 to about 20, andin some embodiments from about 5 to about 15, e.g., about 9.

In some embodiments, the coupling length may range from about 0.1 mm toabout 3.8 mm, in some embodiments from about 0.2 mm to about 3 mm, insome embodiments from about 0.3 mm to about 2.8 mm, and in someembodiments from about 0.3 mm to about 1.5 mm. For example, in someembodiments, the coupling length may be approximately equal to 0.9 mm,corresponding to λ/4 for about 28 GHz and a dielectric material having adielectric constant of about 9 as determined in accordance with ASTMD2520-13 at an operating temperature of 25° C. and frequency of 28 GHz.

The parallel portions of the microstrips may be spaced apart by a gapdistance. The gap distance may be selected to obtain the desiredperformance characteristics of the coupler e.g., (desired impedance) andthe specific component materials implemented. The gap distance may rangefrom about 50 micrometers (μm) to about 750 μm, in some embodiments fromabout 100 μm to about 700 μm, in some embodiments from about 300 μm toabout 600 μm, e.g., 200 μm.

The microstrips may also have a width ranging from about 50 μm to about500 μm, in some embodiments from about 100 μm to about 400 μm, in someembodiments from about 200 μm to about 300 μm, e.g., about 250 μm.

The micostrips may be shielded by one or more ground planes. The groundplane(s) (e.g., base ground plane and/or cover ground plane) may besubstantially parallel to the microstrips and spaced apart by thethicknesses of the substrate(s) (e.g., the base substrate and/or coversubstrate). The thicknesses of the substrates may range from about 50 μmto about 500 μm, in some embodiments from about 100 μm to about 400 μm,in some embodiments from about 200 μm to about 300 μm, e.g., about 250μm. In some embodiments, the thicknesses of the substrate(s) may beselected to be approximately equal to the width of the microstrips.

The vias may also be configured to contribute to the excellent couplingcharacteristics of the coupler. The vias may connect respective ends ofthe microstrips (e.g., on the top surface of the base substrate) withrespective contact pads (e.g., on the bottom surface of the basesubstrate). As noted above, the vias may be formed through the basesubstrate. Thus, the lengths of the vias may be equal to the thicknessof the base substrate. The lengths of the vias may be selected to beequal or proportional to A/4, which may contribute to the excellentcoupling characteristics of the coupler.

In some embodiments, the coupler may include at least one adhesion layerin contact with the thin-film microstrips. The adhesion layer may be orinclude a variety of materials that are suitable for improving adhesionbetween the thin-film microstrips and adjacent layers, such as the basesubstrate, cover substrate, and/or protective layer (e.g., polymericlayer). As examples, the adhesion layer may include at least one of Ta,Cr, TaN, TiW, Ti, or TiN. For instance, the adhesive layer may be orinclude tantalum (Ta) (e.g., tantalum or an oxide or nitride thereof).Without being bound by theory, the material of the adhesion layer may beselected to overcome phenomena such as lattice mismatch and residualstresses.

The adhesion layer(s) may have a variety of suitable thicknesses. Forexample, in some embodiments, the thicknesses of the adhesion layer(s)may range from about 100 angstroms to about 1000 angstroms, in someembodiments from about 200 angstroms to about 800 angstroms, in someembodiments from about 400 angstroms to about 600 angstroms.

The coupler may have a compact form. For example, in some embodiments,each of a length and a width of the coupler may be less than about 5.0mm, in some embodiments less than about 3.5 mm, and in some embodimentsless than about 2.5 mm. For example, in some embodiments each of thelength and width of the coupler may range from about 0.5 mm to about 5mm, in some embodiments from about 1 mm to about 4 mm, and in someembodiments from about 2.5 mm to about 3.5 mm.

Despite the compact form of the coupler, in some embodiments, thecoupler may be configured for grid array type mounting. For example,vias may facilitate grid array type mounting of the coupler to a PCB.Exemplary types of grid array mounting include ball grid array and landgrid array. For example, in some embodiments, the vias may beelectrically connected with respective contact pads disposed on a bottomsurface of the coupler. A base ground plane may also be disposed on thebottom surface of the coupler. The contact pads and base ground planemay provide electrical connections on the bottom surface of the basesubstrate such that the coupler may be surface mounted using grid arraytype mounting.

Grid array type mounting may provide significant advantages. Forexample, mounting the coupler to a PCB may be performed moreefficiently, reducing assembly cost. Additionally, manufacturing costsassociated with forming the high frequency surface mountable coupler maybe reduced compared with alternative mounting options. In someembodiments, the described configuration may also protect electricalcontacts (e.g., contact pads and base ground plane) from electricallyshorting because all electrical contacts may be obscured and protectedunderneath the coupler or underneath the cover substrate. Additionally,the base ground plane may provide for attachment along a significantportion of the total area of the coupler. This may result in a morerobust physical connection between the coupler and the PCB.

FIG. 1A illustrates a top down view of one embodiment of a thin filmcoupler 100 in accordance with aspects of the present disclosure. FIG.1B illustrates a cross section view along section A-A in FIG. 1A. Thethin film coupler 100 may include a base substrate 102 and a pair ofmicrostrips 104. The base substrate may have a top surface 106, and themicrostrips 104 may be formed on the top surface 106 of the basesubstrate 102. The base substrate 102 may also have a length(represented by L_(s) in FIG. 1A) in a longitudinal direction 103, and awidth (represented by W_(s) in FIG. 1A) in a lateral direction 105 thatis perpendicular to the longitudinal direction 103. The base substrate102 may also have a thickness (represented by T_(s) in FIG. 1A) in a Zdirection 107 that is perpendicular to each of the longitudinaldirection 103 and the lateral direction 105.

One of the microstrips 104 may function as a transmission line, and theother of the microstrips 104 may function as a coupled line. At least aportion of the microstrips 104 may be arranged substantially parallel toeach other along a coupling length (represented by L_(c) in FIG. 1A). Insome embodiments, the coupling length, L_(c,) may be proportional toλ/4, where λ represents the desired operating frequency of the coupler.The microstrips 104 may also have a width (represented by W_(m) in FIG.1A). The microstrips 104 may be arranged such that a gap (represented byG_(m) in FIG. 1A) is formed between the microstrips 104. Theelectromagnetic interaction between the microstrips 104 may cause asignal through one of the microstrips 104 (the transmission line) toinduce a signal in the other of the microstrips 104 (the coupled line).

A cover substrate 108 may overlay each of the top surface 106 of thebase substrate 102 and the microstrips 104 that are formed on the topsurface 106. The cover substrate 108 is omitted from FIG. 1A for claritysuch that the top surface 106 of the base substrate 102 is visible. Thecover substrate 108 is visible in FIG. 1B, however. The cover substrate108 may protect the microstrips 104 from electrical short by contactwith other electrically conductive objects. The cover substrate 108 mayfurther protect the microstrips 104 from damage by abrasion or impact.Additionally, the cover substrate 108 may provide a suitable locationfor marking the coupler 100, including, for example, performancespecifications/characteristics, port labels, etc.

Vias 114 may be formed through the base substrate 102 from the topsurface 106 to the bottom surface 110. The vias 114 may electricallyconnect respective ends of the pair of microstrips 104 with contact pads116 formed on the bottom surface 110 of the base substrate 102. Forexample, each microstrip 104 may have an input end and an output endthat is electrically connected with a respective via 114. The vias 114may electrically connect the input end or output end of one of themicrostrips 104 to respective contact pads 116 such that the contactpads 116 act as electrical ports for the coupler 100.

Although not illustrated in FIGS. 1A and 1B, it should be understoodthat the contact pads 116 may configured for grid array mounting. Forexample, a conductive material may be disposed on the contacts pads 116for land grid array or ball grid array type surface mounting. Theconductive material may comprise copper, nickel, aluminum, palladium,gold, silver, platinum, lead, tin, alloys of these materials, or anyother suitable conductive substance suitable as a solder material.During installation the coupler 100 may be arranged in the desiredlocation and heated such that the solder melts forming electricalconnections with the mounting surface (e.g., PCB).

A base ground plane 112 may be formed on the bottom surface 110 of thebase substrate 102. As such, the base ground plane 112 may be spacedapart from the microstrips 104 by the thickness (represented by T_(s) inFIG. 1A) of the base substrate 102. The base ground plane 112 may begenerally parallel with the microstrips 104. The thickness of the basesubstrate 102 may be selected to produce the desired shielding effect onthe microstrips 104 and/or selected to produce the desired responsecharacteristics of the coupler 100.

The base ground plane 112 may be located between the vias 114 in thelongitudinal direction 103 (or lateral direction 105) such that thecontact pads 116 are adjacent corners of the base ground plane 112.Additionally, in some embodiments, the base ground plane 112 may extendsubstantially all of the length or width of the base substrate 102. Forexample, the base ground plane 112 may be sized such that the groundplane 112 can extend between contact pads 116 without directlycontacting the contact pads 116. For example, in some embodiments, theground plane 112 may extend substantially to an edge of the bottomsurface 110 of the base substrate 102. This may provide a larger surfaceby which the coupler 100 can be surface mounted, thereby providing amore mechanically robust attachment.

FIGS. 2A and 2B illustrate perspective views of the embodiment of thethin film coupler described above with reference to FIGS. 1A and 1B. Thecover substrate 108 is omitted for clarity. Only the bottom surface ofthe base substrate 102 is shown for clarity.

FIG. 3A illustrates a top down view of another embodiment of a thin filmcoupler 100 including a cover ground plane 118, according to aspects ofthe present disclosure. The cover substrate 108 is omitted for clarity.FIG. 3B illustrates a section view along section A-A in FIG. 3A. FIG. 3Cillustrates a perspective view in which the cover substrate 108 isomitted for clarity. Only the bottom surface 110 of the base substrate102 is shown.

The coupler 100 may generally be configured as described above withreference to FIGS. 1A, 1B, 2A, and 2B. In addition, the coupler 100 mayinclude the cover ground plane 118, which may be formed on the topsurface 120 of the cover substrate 108. The cover substrate 108 may havea thickness (represented in FIG. 3B as T_(cover)) in the Z direction107. As such, the cover ground plane 118 may be spaced apart from themicrostrips 104 in the Z direction 107 by the thickness of the coversubstrate 108. The cover ground plane 118 may be generally parallel withthe microstrips 104 and/or the base ground plane 112. The thickness ofthe cover substrate 108 may be selected to produce the desired shieldingeffect on the microstrips 104 and/or selected to produce the desiredresponse characteristics of the coupler 100.

The cover ground plane 118 may be electrically connected with the baseground plane 112 in a variety of ways. For example, in some embodiments,one or more ground vias 122 may extend through the base substrate 102and the cover substrate 108 and electrically connect the cover groundplane 118 with the base ground plane 112. For example, a pair of groundvias 122 may be symmetrically arranged on either side of the microstrips104. In other embodiments, a single ground via 122 may electricallyconnect the cover ground plane 118 with the base ground plane 112.

In other embodiments, however, the cover ground plane 118 may beelectrically connected through a side wall connection, castellation typeconnection, or any other suitable type of electrical connection to thebase ground plane 112 and/or the bottom surface 110 of the basesubstrate 102. For example, a side wall connection may be formed thatextends from the cover ground plane 118 to the base ground plane 112 orthe bottom surface 110 of the base substrate 102. The side wallconnection may be formed by sputtering or any other suitable depositiontechnique on a surface of the base substrate 102 and/or cover substrate108 that extends in the Z direction 107. As another example, acastellation type connection may be formed by drilling holes in thesubstrates 102, 108 along dicing lines before the substrates 102, 108are diced along the dicing lines to form individual couplers 100. Aconductive layer may be formed on the resulting castellation surfaceusing sputtering or any other suitable deposition technique.

The cover ground plane 112 may provide several advantages. For example,the cover ground plane 112 may provide additional electromagneticshielding of the microstrips 104. This may reduce the amount ofelectromagnetic interference from ambient radio waves, for example.

It should be understood that additional combinations and variations arepossible based on the disclosed embodiments. For example, the couplermay be partially configured for grid array-type mounting. For instance,a single via or a pair of vias may connect one or two ends of themicrostrips with respective contact pads on the bottom surface of thebase substrate. The other ends of the microstrips may be electricallyconnected using any other suitable means, including conductivecastellations, soldered wires, etc.

FIG. 4 illustrates diagrammatic views of various surfaces of severalembodiments of the coupler in accordance with aspects of the presentdisclosure. For each embodiment, labeled A-H, the shaded portions of“PAD1” represent thin film layers on the bottom surface of the basesubstrate that form the base ground plane and contact pads. The shadedportions of “PAD2” represent thin film layers on the top surface of thebase substrate that form the microstrips. “VIA” illustrates thelocations of the vias in the base substrate. The shaded portions of“LGA” illustrate the shapes of a layer of conductive material formedover the base ground plane and contact pads to facilitate land gridarray-type surface mounting of the coupler.

Referring to FIG. 5, aspects of the present disclosure are directed to amethod 200 for making a high frequency coupler configured for array typesurface mounting. In general, the method 200 will be described hereinwith reference to the thin film coupler 100 described above withreference to FIGS. 1-3. However, it should be appreciated that thedisclosed method 200 may be implemented with any suitable thin filmcoupler. In addition, although FIG. 5 depicts steps performed in aparticular order for purposes of illustration and discussion, themethods discussed herein are not limited to any particular order orarrangement. One skilled in the art, using the disclosures providedherein, will appreciate that various steps of the methods disclosedherein can be omitted, rearranged, combined, and/or adapted in variousways without deviating from the scope of the present disclosure.

Referring to FIG. 5, the method 200 may include, at (202), forming atleast one via through hole that extends from a top surface of amonolithic base substrate to a bottom surface of the monolithic basesubstrate. For example, the through hole(s) may be formed using laserdrilling or any other suitable method. Additionally, it should beunderstood that multiple couplers may be formed on a base substratesheet, which may be diced to form individual couplers.

The method may include, at (204), depositing a first thin filmmicrostrip and a second thin film microstrip on a top surface of amonolithic base substrate. For example, in some embodiments, an additivetechnique (e.g., sputtering, electroless plating, etc.) may be used toform a layer of suitable thickness of a conductive metal on the basesubstrate. A lithographic screen may be deposited over the conductivemetal layer. Portions of the copper layer may then be removed using anysuitable etching technique to produce the desired pattern for themicrostrips. A second etching step may then be used to remove thelithographic screen. A variety of suitable etching techniques may beused, including dry etching using a plasma of a reactive gas (e.g.,oxygen, chlorine, boron trichloride).

Each of the first and second thin film microstrips may be sized andspaced apart to provide the desired performance characteristics. Forexample, the coupler may have a coupling factor that is greater thanabout −30 dB from about 18 GHz to about 32 GHz and/or have a couplingfactor that is greater than about −30 dB at about 28 GHz. The couplinglength (represented by L_(c) in FIG. 1A) along which the microstrips aresubstantially parallel to each other may be sized to produce the desiredcharacteristics. For example, in some embodiments, this length may beequal to about λ/4 or λ/8, where A represents the desired operatingfrequency of the coupler (e.g., 28 GHz), as discussed above.

The method may include, at (206), depositing a conductive via materialinside the at least one via through hole to form at least one viaelectrically connecting at least one of the first thin film microstripor the second thin film microstrip with a contact pad on the bottomsurface of the monolithic base substrate. For example, in someembodiments, depositing the conductive via material may includesputtering, electroless plating, or any other suitable thin filmdeposition process. In some embodiments, this may be performedconcurrently with the deposition of the microstrips, at (204).

In some embodiments, the method 200 may also include forming a baseground plane on a bottom surface of the monolithic base substrate. Thebase ground plane may be formed in a similar manner as described aboveregarding forming the microstrips, at (204). In some embodiments, thebase ground plane may be formed during the same thin film formationsteps (e.g., deposition, lithography, etching) described above that maybe used to form the microstrips, at (204). In some embodiments, themethod 200 may also include forming a layer of conductive material(e.g., solder, tin, lead, gold, alloys thereof or any other suitableconductive material) over the base ground plane and/or contact pads onthe bottom surface of the monolithic base substrate that is suitable forgrid array-type mounting.

FIG. 6A illustrates a diagrammatic view of an embodiment of a coupler300 having a transmission line 302 and a coupled line 304. Signal may beinput at port 1 and directly transmitted to port 2 of the transmissionline 302. The coupler 300 produces a coupled signal in the coupled line304 at port 3. Port 4 is often grounded.

FIG. 6B represents theoretically calculated S-parameters for anembodiment of a coupler in accordance with aspects of the presentdisclosure across a frequency range extending from 18 GHz to 32 GHz. Asis understood in the art, the S-parameters are expressed with subscriptsin the following form: S_(ab). The subscript values, a and b, indicateport numbers associated with the S-parameter such that each S-parametercan understood to represent the signal resulting at port a as a resultof signal input at port b. As is understood in the art, the S-parametersare commonly referred to as follows:

S-Parameter Name S₁₁ Return Loss S₂₁ Insertion Loss S₃₁ Coupling FactorS₄₁ Isolation Factor

Referring to FIG. 4B, the theoretical coupling factor, S₃₁, is greaterthan about −30 dB from about 18 GHz to about 32 GHz, and in someembodiments, greater than about −25 dB from about 18 GHz to about 32GHz. The theoretical coupling factor at 28 GHz is −19.84 dB, which isgreater than −20 dB.

Additionally, the theoretical return loss, S₁₁, is less than about −30dB from about 22 GHz to about 32 GHz. The theoretical return loss, S₁₁,is also less than −15 dB from about 18 GHz to about 32 GHz. A low returnloss is generally desirable because return loss represents the portionof the signal that is reflected back to the source port (Port 1).

The insertion loss, S₂₁, is greater than about −0.75 dB from about 18GHz to about 32 GHz. An insertion loss close to zero indicates that themagnitude of the signal is substantially unaffected at Port 2 as aresult of the coupler. Lastly, the theoretical isolation factor, S₄₁, isless than about −30 dB from about 18 GHz to about 32 GHz, and less thanabout −45 dB from about 22 GHz to about 28.5 GHz. Application

The disclosed coupler may find particular application in a circuit of abase station that is adapted for 5G frequencies or associatedinstrumentation or equipment. Additional applications can includesmartphones, signal repeaters (e.g., small cells), relay stations,radar, radio frequency identification (RFID) devices, and any othersuitable device that employs high frequency radio signals.

The base station circuit may be configured to transmit, receive, orotherwise process 5G radio signals. The base station circuit may includea signal source component, such as a radio frequency transmitter,receiver, or component thereof (e.g., mixer, amplifier, modulator,etc.). The coupler may be operatively connected with the signal sourcecomponent. The coupled line may be used to provide a coupled signal to aseparate component (e.g., for monitoring or control of the signal sourcecomponent). For example, the coupled line may provide a coupled signalto a feedback control loop associated with an amplifier of a radiofrequency transmitter.

EXAMPLE

The ability to form a compact, high frequency coupler having a couplingfactor of greater than about −30 dB at about 28 Ghz and that is suitablefor grid array surface mounting was demonstrated.

As is known in the art, the case size of electronic devices may beexpressed as a four digit code (e.g., 2520), in which the first twodigits are the length of the device in millimeters (or in hundredths ofan inch) and the last two digits are the width of the device inmillimeters (or in hundredths of an inch). Common metric case sizes mayinclude 2012, 1608, and 0603.

A 3216 metric case size (1206 imperial case size) coupler was produced.The coupler (and base substrate) have a length of about 3.2 mm (0.125inch) and a width of about 1.6 mm (0.06 inch). Referring back to FIG.1A, the coupling length (represented by L_(c)) is about 1.25 mm. The gap(represented by G_(m) in FIG. 1A) formed between the microstrips isabout 500 micrometers. The width (represented by W_(m) in FIG. 1A) ofthe microstrips is about 254 micrometers. The vias are about 300micrometers in diameter. The microstrips and base ground plane are eachabout 3 micrometers thick. The base substrate is about 254 micrometersthick.

Coupling, return loss, insertion loss, and directivity were measuredfrom 27.2 GHz to 29.8 GHz:

TABLE 1 S-Parameters for 3216 metric case size (1206 imperial case size)coupler Frequency Coupling Return Loss Insertion Directivity (GHz) (dB)(dB) Loss (dB) (dB) 27.2 −18.9 −14.2 −0.32 13.5 27.6 −19.3 −17.1 −0.5316.3 28 −18.8 −21.2 −0.36 19.9 29 −18.2 −19.6 −0.16 18.7

As shown in Table 1, the 1206 imperial case size coupler has a couplingfactor that is −19.84 dB at 28 GHz, which is greater than about −30 dB.

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is:
 1. A high frequency, surface mountable coupler comprising: a monolithic base substrate having a top surface, a bottom surface, a length in a longitudinal direction, and a width in a lateral direction that is perpendicular to the longitudinal direction; a first thin film microstrip disposed on the top surface of the monolithic base substrate, the first microstrip having an input end and an output end; a second thin film microstrip disposed on the top surface of the monolithic base substrate, the second microstrip having an input end and an output end; and at least one via extending through the monolithic base substrate from the top surface to the bottom surface of the monolithic base substrate, the at least one via electrically connected with at least one of the input end or the output end of the first microstrip or the second microstrip; wherein the coupler has a coupling factor that is greater than about −30 dB at about 28 GHz.
 2. The coupler of claim 1, wherein the coupler has a coupling factor that is greater than about −30 dB from about 10 GHz to about 70 GHz.
 3. The coupler of claim 1, further comprising at least one contact pad disposed on the bottom surface of the base substrate, the at least one contact pad electrically connected with the at least one via.
 4. The coupler of claim 1, further comprising a base ground plane disposed on the bottom surface of the monolithic base substrate.
 5. The coupler of claim 1, wherein each of the length and the width of the base substrate is less than about 7 mm.
 6. The coupler of claim 1, wherein coupler further comprises four contact pads disposed on the bottom surface of the base substrate, and wherein the at least one via comprises: a first via electrically connecting the input end of the first thin film microstrip to a first of the four contact pads; a second via electrically connecting the output end of the first thin film microstrip to a second of the four contact pads; a third via electrically connecting the input end of the second thin film microstrip to a third of the four contact pads; and a fourth via electrically connecting the output end of the second thin film microstrip to a fourth of the four contact pads.
 7. The coupler of claim 1, wherein the coupler is configured for grid array type mounting.
 8. The coupler of claim 1, wherein the base substrate comprises a ceramic material.
 9. The coupler of claim 1, wherein the base substrate comprises sapphire.
 10. The coupler of claim 1, further comprising a cover substrate arranged on the top surface of the base substrate.
 11. The coupler of claim 10, further comprising a cover ground plane disposed on a top surface of the cover substrate.
 12. The coupler of claim 10, wherein the cover ground plane is electrically connected with the base ground plane.
 13. The coupler of claim 12, wherein the at least one via comprises a ground via extending through each of the base substrate and the cover substrate, the ground via electrically connecting the cover ground plane with the base ground plane.
 14. The coupler of claim 1, wherein the at least one via comprises a pair of vias, and wherein the base ground plane extends between the pair of vias in one of the longitudinal or lateral directions.
 15. The coupler of claim 1, further comprising a polymeric protective layer exposed along an exterior of the coupler.
 16. The coupler of claim 1, further comprising an adhesion layer in contact with at least one of the first thin film microstrip or the second thin film microstrip.
 17. A method for forming a high frequency, surface mountable coupler, the method comprising: forming at least one via through hole that extends from a top surface of a monolithic base substrate to a bottom surface of the monolithic base substrate; depositing a first thin film microstrip and a second thin film microstrip on the top surface of the monolithic base substrate, each of the first and second thin film microstrips being sized and spaced apart such that the coupler has a coupling factor that is greater than about −30 dB at about 28 GHz; and depositing a conductive via material inside the at least one via through hole to form at least one via electrically connecting at least one of the first thin film microstrip or the second thin film microstrip with a contact pad on the bottom surface of the monolithic base substrate.
 18. The method of claim 17, further comprising forming a base ground plane on a bottom surface of the monolithic base substrate.
 19. The method of claim 17, wherein forming the at least one via through hole comprises laser drilling the at least one via through hole.
 20. The method of claim 17, further comprising arranging a cover substrate on top of the top surface of the base substrate.
 21. The method of claim 20, further comprising depositing a cover ground plane on a top surface of the cover substrate.
 22. The method of claim 17, further comprising forming at least one ground via through hole that extends from the top surface of the cover substrate to the bottom surface of the base substrate.
 23. A high frequency, surface mountable coupler comprising: a monolithic base substrate having a top surface, a bottom surface, a length in a longitudinal direction, a width in a lateral direction that is perpendicular to the longitudinal direction, and a thickness in a Z direction that is perpendicular to each of the longitudinal direction and the lateral direction; a first thin film microstrip disposed on the top surface of the monolithic base substrate, the first microstrip having an input end and an output end; a second thin film microstrip disposed on the top surface of the monolithic base substrate, the second microstrip having an input end and an output end; and wherein at least a portion of the first thin film microstrip and at least a portion of the second thin film microstrip extend parallel to each other in a first direction along a coupling length, and wherein the coupling length ranges from about 0.2 mm to about 3.8 mm.
 24. The coupler of claim 23, wherein each of the first thin film microstrip and the second thin film microstrip have respective widths in a second direction that is perpendicular to the first direction, and each of the widths of the first and second thin film microstrips range from about 50 micrometers to about 500 micrometers.
 25. A base station circuit comprising: a signal source component; a high frequency, surface mountable coupler operatively connected with the signal source component, the coupler comprising: a monolithic base substrate having a top surface, a bottom surface, a length in a longitudinal direction, and a width in a lateral direction that is perpendicular to the longitudinal direction; a first thin film microstrip disposed on the top surface of the monolithic base substrate, the first microstrip having an input end and an output end; a second thin film microstrip disposed on the top surface of the monolithic base substrate, the second microstrip having an input end and an output end; and at least one via extending through the monolithic base substrate from the top surface to the bottom surface of the monolithic base substrate, the at least one via electrically connected with at least one of the input end or the output end of the first microstrip or the second microstrip; wherein the coupler has a coupling factor that is greater than about −30 dB at about 28 GHz. 